Terahertz emitters are important for fundamental studies in an interesting frequency regime and for applications ranging
from medical diagnostics to see-through imaging. A simple approach to THz emission from semiconductors is based on
intracenter optical transitions in dopants and impurities in semiconductors. The centers can be excited either electrically
or optically, and the THz emission occurs when carriers in the dopant upper energy states relax toward the ground state.
Both n–type and p–type dopants as well as deep impurities can be used for THz emission from many host
semiconductors including silicon, SiC, and GaN. Unlike with conventional p–n junction devices, the centers for THz
emission must be occupied and not thermally ionized, which suggests the need for deep energy levels and/or low
temperature operation. Significant center occupation at elevated temperatures favors the wide bandgap semiconductors
such as SiC and GaN, in which the dopant ionization energy can greatly exceed the thermal energy kBT at room
temperature. For example, electrically pumped THz emitters fabricated from nitrogen-doped SiC can operate at
temperatures to about 250 K in pulse mode. The SiC emission spectra had peaks from 5 to 12 THz (20 to 50 meV), and
these surface-emitting devices produced a peak power density of 30 milliwatt-cm-2 at 77 K, which is suitable for a wide
range of high power THz applications. We report the characteristics and limitations of electrically pumped dopanttransition
THz emitters, and their performance in several semiconductor systems.
A fabrication process of three-dimensional Woodpile photonic crystals based on multilayer photolithography
from commercially available photo resist SU8 have been demonstrated. A 6-layer, 2 mm × 2mm woodpile has been
fabricated. Different factors that influence the spin thickness on multiple resist application have been studied. The
fabrication method used removes, the problem of intermixing, and is more repeatable and robust than the multilayer
fabrication techniques for three dimensional photonic crystal structures that have been previously reported. Each layer is
developed before next layer photo resist spin, instead of developing the whole structure in the final step as used in
multilayer process. The desired thickness for each layer is achieved by the calibration of spin speed and use of different
photo resist compositions. Deep UV exposure confinement has been the defining parameter in this process. Layer
uniformity for every layer is independent of the previous developed layers and depends on the photo resist planarizing
capability, spin parameters and baking conditions. The intermixing problem, which results from the previous layers left
uncrossed linked photo resist, is completely removed in this process as the previous layers are fully developed, avoiding
any intermixing between the newly spun and previous layers. Also this process gives the freedom to redo every spin any
number of times without affecting the previously made structure, which is not possible in other multilayer process where
intermediate developing is not performed.
Infrared semiconductor ring laser fabrication typically involves planarization of ridge waveguide device structures and deposition of metal electrodes for electrical pumping. Uniform planarization across large samples is difficult to achieve. This leads to inadequate electrical contact between portions of the ring resonator and the deposited metal electrode layer whereby the devices are not optimally pumped. This can lead to high threshold currents and device
failure. The problem of inadequate electrical pumping on account of non-idea planarization has been addressed by utilizing a metallic etch mask instead of the commonly used photoresist 'soft' mask. The metallic mask remains intact after ridge etching and the other ensuing fabrication steps to form a continuous metallic cover above the entire device structure. This metallic cover ensures proper electrical contact between the ring resonator and the deposited
metal electrode layer even when planarization imperfections render only certain portions of the resonator in proper electrical contact with the metal electrode layer. The proposed fabrication process has led to large diameter ring lasers with high yield and low threshold current levels. These devices are robust and exhibit stable operation over large current ranges in addition.